Nitrate and nitrite microgradients in barley rhizosphere as detected by a highly sensitive denitrification bioassay

• Published in: Applied and Environmental Microbiology Vol. 58; no. 8; pp. 2375 - 2380
• Main Authors: Binnerup, S.J. (The Royal Veterinary and Agricultural University, Frederiksberg C, Denmark), Sorensen, J
• Format: Journal Article
• Language: English
• Published: Washington, DC American Society for Microbiology 01.08.1992
• Subjects: Agronomy. Soil science and plant productions; Bacteria; Biological and medical sciences; Biological Assay - methods; Biotechnology; Cellular biology; DENITRIFICACION; DENITRIFICATION; Evaluation Studies as Topic; Fundamental and applied biological sciences. Psychology; General agronomy. Plant production; Generalities. Analysis and diagnosis methods; Hordeum; HORDEUM VULGARE; NITRATE; Nitrates - metabolism; NITRATOS; NITRITE; Nitrites - metabolism; NITRITOS; Plants - metabolism; PSEUDOMONAS AERUGINOSA; Pseudomonas aeruginosa - metabolism; RHIZOSPHERE; RIZOSFERA; Soil - analysis; Soil-plant relationships. Soil fertility. Fertilization. Amendments; Soils; Pseudomonadales; Monocotyledones; Hordeum vulgare; Nitrites; Nitrates; Cereal crop; Rhizosphere; Soils; Gramineae; Angiospermae; Analytical method; Denitrification; Bacteria; Pseudomonadaceae; Pseudomonas aeruginosa; Spermatophyta; Detection; Quantitative analysis
• ISSN: 0099-2240; 1098-5336
• DOI: 10.1128/aem.58.8.2375-2380.1992

A highly sensitive denitrification bioassay was developed for detection of NO3(-1) and NO2(-1) in rhizosphere soil samples. Denitrifying Pseudomonas aeruginosa ON12 was grown anaerobically in citrate (30 mM) minimal medium with KClO3 (10 mM) and NaNO2 (3 mM), which gave cells capable of NO2(-1) reduction to N2O but incapable of NO3(-1) reduction to NO2(-1). Growth on citrate minimal medium further resulted in the absence of N2O reduction. When added to small soil samples in O2-free vials, such cells could be used to convert the indigenous NO2(-1) pool to N2O, which was subsequently quantified by gas chromatography. Cells grown in KClO3-free citrate medium with 10 mM NaNO3 as the electron acceptor were capable of reducing both NO3(-1) and NO2(-1), and these cells could subsequently be added to the sample to convert the indigenous NO3(-1) pool to N2O. Concentrations of both NO3(-1) and NO2(-1) were thus determined as N2O, with a detection limit of approximately 10 pmol of N. The bioassay could be used to determine NO3(-1) and NO2(-1) pools in 10-mg soil samples taken along a microgradient in the rhizosphere of field-grown barley plants. At both low (10%, wt/wt) and high (18%, wt/wt) water content, relatively high levels of NO2(-1) were found in the rhizosphere compared with bulk soil. Under dry conditions, NO3(-1) was also more abundant in the rhizosphere than in the bulk soil, whereas such a difference was not observed at the high water content. The roles of plant metabolism and bacterial nitrification and denitrification processes for NO3(-1) and NO2(-1) availability in the rhizosphere are discussed